The most common AV programs for Linux are Clam or Klam (KDE) and AVG from
Grisoft. They do not give you the ability to scan as you download or open email
though. You need to set it up using dazuko and this is complicated for the
ability to scan incoming traffic. Otherwise you need to manually scan files or
to scan your home folder occasionally.
I have used Linux for many years without even having an AV installed. It is not
an issue for me, but I am not hyper about such thngs.
If I wanted to be more secure, I would run Windows apps from Virtual Box which I
do from time to time. This means that any Windows files are relegated to the VM
and cannot contaminate my main machine. I do not file share so it is not an big
deal for me.
I have a shared folder where I can put files from either Windows or Ubuntu. I
can access my drives, usb devices, sound card or network card from either. It is
the best of both worlds. If anyone compromises Windows, it is on its own virtual
hard drive that can be reformatted without any pain. I run AVG on Windows, but
nothing on Linux. I tend to do my email in Ubuntu using Thunderbird or Firefox
for webmail. I could easily do it from the Windows VM.
You can also get VMs that are hardened for the security paranoid. You could run
everything that goes out to the world from such a VM and this would give an
added measure of security. Linux security is not perfect, but with its
permissions setup and with few executable binaries it is better than Windows by
far. You have lots of options depending on the length that you are willing to
go.